Optical modules include, for example, an optical waveguide device that uses an electro-optical crystal substrate such as a LiNbO3 (LN) substrate and a LiTaO2 substrate or a semiconductor substrate such as a GaAs substrate and an InP substrate. The optical waveguide device has an optical waveguide formed by forming and thermally diffusing a metal film of titanium (Ti), etc. on a portion of the substrate. Alternatively, the optical waveguide is formed by proton exchange in benzoic acid after patterning. Subsequently, electrodes can be disposed near the optical waveguide to form an optical module such as an optical modulator.
If such an optical modulator is driven at high speed, terminals of a signal electrode and a ground electrode are connected by a resistor to form a traveling-wave electrode and a high-speed microwave signal (electronic signal) is applied to an RF terminal from the input side. In this case, an electric field changes refraction indexes of a pair of parallel waveguides A and B toward +Δ and −Δ, respectively, and a phase difference is changed between the parallel waveguides A and B. As a result, a signal light modulated in intensity is output from an exist waveguide due to Mach-Zehnder interference.
High-speed optical response characteristics can be obtained by matching speeds of light and the high-speed microwave signal (electronic signal). After passing through the optical modulator, the electronic signal passes through a capacitor and is terminated by a termination resistor. An electrode is divided before the capacitor and one branch is connected through a bias resistor to a DC terminal while the other is terminated by the termination resistor. This configuration acts as bias and, when voltage is applied to the DC terminal, a bias point and drive voltage of a Mach-Zehnder unit can be controlled.
Such an optical modulator has a Mach-Zehnder modulating unit and a relay substrate to which an electronic signal for driving the Mach-Zehnder unit is input. The relay substrate relates to, for example, a technique of disposing the Mach-Zehnder modulating unit between a signal input substrate and a signal termination substrate equipped with a termination resistor (see, e.g., Japanese Laid-Open Patent Publication No. 2004-226769) and a technique of mounting a signal input circuit and a signal termination circuit on one circuit substrate (see, e.g., Japanese Laid-Open Patent Publication No. H5-289034).
Recently, optical communication is further multivalued and polarization-multiplexed so as to achieve a larger capacity and the configuration of modulators is increasingly complicated. For example, also in a modulator, a modulation mode is used that generates a signal multivalued and polarization-multiplexed by disposing two sets of Mach-Zehnder modulating units having a pair of parallel waveguides and inputting independent signals to the two sets of the Mach-Zehnder modulating units.
However, in a configuration having two sets of the Mach-Zehnder modulating units, the number of signal paths of electronic signals is doubled on the substrate of the Mach-Zehnder modulating units and a given space is required for layout of the signal paths. The numbers of RF terminals, DC terminals, capacitors, bias resistors, and termination resistors are accordingly doubled on a relay substrate and space is required for mounting. Therefore, the relay substrate is increased in size, for example, in the length direction along the parallel waveguides of the Mach-Zehnder modulating units, causing a problem of an increased size of a module that includes the modulator.